Because hydrogen fuel (hydrogen energy) does not discharge CO2 at the time of an oxidation reaction, it is expected to be used in the near future as a new type of energy in place of conventional fossil fuels. Hydrogen has a low boiling point, and when it is stored at a low temperature, a thermally insulated vessel is necessary. When it is stored as a gas, a high-pressure tank is necessary. For example, the pressure of gas cylinders for hydrogen vehicles which store hydrogen as a gas is currently set at 35 MPa.
The density of liquid hydrogen at atmospheric pressure is greater than the density of compressed hydrogen gas under 35 MPa at room temperature. Therefore, liquid hydrogen has the advantage over gaseous hydrogen that it has a higher storage density. In this manner, liquid hydrogen is an effective form of utilizing hydrogen fuel.
MgB2, which is a new superconductor which was discovered in Japan in 2001, has a critical temperature of 39 K. Therefore, it can exhibit superconductivity in liquid hydrogen, which has a boiling point of approximately 20 K under atmospheric pressure. Liquid hydrogen fluid level gauges which can measure the amount of liquid hydrogen remaining inside a storage vessel from the exterior thereof and which use MgB2-based superconductors as liquid hydrogen fluid level sensing elements (referred to below simply as sensors) have been proposed.
Patent Document 1 discloses a fluid level sensing element for detecting the level of liquid hydrogen. It comprises an MgB2 superconducting wire which is in a superconducting state at the boiling point of liquid hydrogen and a tubular metal sheath which covers the superconducting wire.
Patent Document 2 discloses a fluid level sensing element for liquefied gas in which superconducting compound layers comprising Mg and B with a molar ratio Mg:B=X:Y of 1:1.2-1.9 or 2.1-2.8 or 2:0.4-0.9 or 1.1-1.8 and at least one of C and Si are continuously disposed in the lengthwise direction of all or a portion of the surface of a wire-shaped metal conductor. At the time of measuring a fluid level, the immersed portion of the Mg/B compound is in a superconducting state.
Patent Document 3 discloses a liquid hydrogen fluid level sensing element having an elongated or coil-shaped superconductor expressed as Mg(B1-xCx)2 in which a portion of the B atoms in MgB2 are replaced by C atoms and a covering metal which covers the surface of the superconductor.
Non-Patent Documents 1-5 disclose that the critical temperature of a pellet made from a superconductor in which Al is added to MgB2 is lowered to below 36 K.